In recent years, demand for high-performance Internet protocol (IP) networks that are able to support a large throughput and ensure high-quality services for a large number of users has been increasing.
One of the solutions is the use of optical networking which provides fast data transmission, enables dynamic resource provisioning, and realizes high scalability. While multiprotocol label switching (MPLS) system has been proposed and standardized by IETF (Internet Engineering Task Force) to increase scalability and flexibility of the whole network, a solution for managing MPLS networks directly in the optical domain using multi protocol wavelength switching or multiprotocol lambda switching (MPL(lamda)S) has been introduced. The MPL(lambda)S is also called a generalized MPLS (GMPLS) when optical codes are used as labels.
On the other hand, the optical code division multiple-access (OCDMA) technology has been receiving increased attention due to its capability of realizing not only a super-high speed and very large capacity communication but also a flexible and secure network with highly confidentiality. At the present, OCDMA based passive optical networks (PON) are receiving increasing interest thanks to their unique features of asynchronous access, fast re-configurability, and high confidentiality. In the OCDMA transmission, every user is provided with a security key which is called an optical code. The optical code is used to encode every bit of communication data, so that multiplexing signals in the same wavelength at the same time is enabled.
In particular, the CDMA technique assigns a specific optical code to each user, code which is independent of the information signal to be transmitted. The encoding operation, called spreading, consists of multiplying the code assigned to each single user by the information signal. Instead, in the decoding operation, the receiver carries out a correlation between the received signal and the code of the user which is intended to be received (despreading). Therefore, in order to avoid interference among the various users simultaneously accessing to the network, it is necessary that the codes are orthogonal with respect to each other.
In both GMPLS system and OCDMA system, the code cardinality, i.e. the number of labels, is one of the issues to be considered. Furthermore, in order to enable accurate performances for routers of a GMPLS network and to ensure accurate detection in an OCDMA system, it is necessary to precisely distinguish the different optical codes, which requires that the peak of the auto-correlation function is as high as possible whereas the cross-correlation function must be close to zero everywhere. In order to enable code generation and processing directly in the optical domain in an accurate, reliable, simple, and inexpensive way, and in order to provide a set of optical codes which are highly orthogonal, a multiport encoder/decoder (E/D) capable of generating/processing N phase shifted keying (PSK) codes simultaneously is disclosed in WO2005/064834 by the inventors of the present invention et al (see e.g. Patent Citation 1).
Also, the inventors of the present invention introduced an E/D having an arrayed waveguide grating (AWG) configuration, and when a single laser pulse is sent into one of the encoder input ports, N optical codes are generated at the encoder outputs (see e.g. Non Patent Citations 1 and 2).
In the above-mentioned Patent Citation 1 and Non Patent Citations 1 and 2, a multidimensional E/D is also disclosed. The multidimensional E/D can increase the code cardinality without increasing the code length. For example, if two or more simultaneous laser pulses are driven into different device inputs, N orthogonal codes are generated. Since each different combination of input pulses generates a different set of N codes, the number of orthogonal codes that the device can generate/process can be largely increased.
According to the above-mentioned Patent Citation 1 and Non Patent Citations 1 and 2, the code cardinality of the set of codes generated by the multidimensional E/D is described as follows:
Called n the number of inputs into which a pulse is sent, with n<N, the cardinality of the set of generated codes increases up to
      [          Math      .                          ⁢      1        ]        (                            N                                      n                      )  
whereas the code length remains equal to N. The maximum number of OCs of length N which may be generated by using a multidimensional configuration is equal to
      [          Math      .                          ⁢      2        ]        (                            N                                                  N            /            2                                )  which is obtained by considering n=N/2 inputs.
By way of example, with N=8 ports, the code cardinality of 4-dimensional OCs is 70. As for a device with N=100 ports, and n=50 input pulses, more than 1029 different codes can be obtained. While the code cardinality of the multidimensional E/D seems to be large enough for a secure communication, it is not so.
In order to describe the reason why, an eavesdropping in a passive optical network will now be considered as an example.
Passive optical networks (PONs) are a cost-effective solution to the growing demand for broadband communication services from residential and business costumers, as they promise very high bit rates, broad application supports and enhanced flexibility.
However, their weak point is the lack of confidentiality, because in standard PONs, the downstream data is broadcasted, and all the optical network units (ONU)s receive the same information from the optical line terminal (OLT).
FIG. 35(1) is a schematic diagram of a PON 900 which is composed of an optical line terminal (OLT) 910, an optical splitter 930 connected to the OLT 910 with an optical fiber 920, and optical network units (ONUs) 950 connected to the optical splitter 930 with optical fibers 940. As shown in FIG. 35(1), the OLT 910 broadcasts the downstream data indicated by solid line arrows. The optical splitter splits the data to be distributed to all the ONUs 950.
FIG. 35(2) is a block diagram of the OLT 910 which is composed of a laser source 911, a modulator 912, and an optical encoder 913. A dotted line arrow represents an electrical signal and a solid line arrow represents an optical signal. When a laser light from the laser source 911 and electric data is provided, the modulator 912 of the OLT 910 modulates the laser light with the data and provides a modulated signal to the encoder 913. The encoder 913 encodes the received signal using a security key provided externally and outputs an encoded signal.
FIG. 35(3) is a block diagram of the ONU 950 which is composed of a decoder 951, a photodetector 952, an electric filter 953, and a thresholder 954. The decoder 951 decodes the signal using a security key and provides the decoded signal to the photodetector 952. Output of the photodetector 952 passes through the electric filter 953 and the thresholder 954 and the data is reproduced. While only the ONUs 950 provided with a matched decoder 951 is supposed to recognize the signal, an eavesdropper who taps the communication as shown in FIG. 35(1) can recognize the signal if the eavesdropper has a matched decoder.
Meanwhile, the CDMA has proven to be a secure transmission technique, since each user encrypts a plaintext message into a ciphertext. Therefore, PONs using electronic and optical CDMA has been considered. Moreover, the optical code division multiplexing (OCDM) has the advantage to encrypt data at a very high data rate, using only passive optical devices that can be also easily reconfigured when the secret key must be updated.
Namely, the OCDM technique can be applied to the OLT 910 and the ONUs 940. By applying the multi-dimensional E/D discussed above, a large code cardinality can be provided.
For example, FIGS. 36(1) and 36(2) respectively shows the multidimensional coding and decoding processes at an OLT 960 and an ONU 970, respectively, where the multi-dimensional E/D is applied.
The OLT 960 shown in FIG. 36(1) is composed of a laser light source 961, two modulators 962 and 963, two 1×N/2 splitters 964 and 965, a nonblocking switch 966, and an E/D 967.
On the other hand the ONU 970 shown in FIG. 36(2) is composed of an E/D 971, photodetectors 972, and an electronic logic circuit 973 for code recognition.
For sake of simplicity, we assume that the first N/2 ports of the encoder are used to transmit a ‘0’, and the remaining ones are used for a logic ‘1’. At the OLT 960, a laser light from the laser light source 961 is provided to the modulators 962 and 963.
Data ‘011100’ and inverted data ‘100011’ are provided to the modulators 962 and 963, respectively, so that laser pulses are provided to the two 1×N/2 splitters 964 and 965. The laser pulses are switched between two 1×N/2 splitters 964 and 965, according to the bit value. The optical nonblocking switch 966 is driven by the security key and connects n ports of the encoder with the input pulses, selecting a n dimensional code.
At the ONU 970, the n ACPs (autocorrelation peaks) detected by the photodetectors 972 at the output ports identify the n-dimensional code and the electronic logic circuit 973 converts this information in the received bit. This encoding technique is known as code shift keying (CSK), in the case of n=2 and it also allows balanced detection to reduce MAI noise.
However, an eavesdropper that possesses a matched decoder can easily intercept the code. Furthermore, a spectral analysis of the received signal, using an optical spectrum analyzer (OSA), can also identify the code, because the n-dimensional codes are superposition of n different frequency subbands.
Although the security the security OCDM transmission should not rely on the coding/decoding processes of a single E/D, because an adversary could be able to find the matched decoder, but it is necessary to introduce more degrees of freedom to prevent that this could happen.    Patent Citation 1: WO2005/064834    Non Patent Citation 1: G. Cincotti, N. Wada, and K.-i. Kitayama ‘Characterization of a full encoder/decoder in the AWG configuration for code-based photonic routers. Part I: modelling and design,’ IEEE J. Lightwave Technol., vol. 24, n. 1, in press 2006.    Non Patent Citation 2: N. Wada, G. Cincotti, S. Yoshima, N. Kataoka, and K.-i. Kitayama ‘Characterization of a full encoder/decoder in the AWG configuration for code-based photonic routers. Part II: experimental results’ IEEE J. Lightwave Technol., vol. 24, n. 1, in press 2006